1. Field of the Invention
The present invention relates to a thermoplastic resin injection molding machine, and more particularly to a thermoplastic resin injection molding machine including a plasticating unit and an injecting unit provided independently of each other.
2. Description of the Related Art
As a conventional injection molding machine including a plasticating unit for plasticating a thermoplastic resin, and an injecting unit for injecting the plasticated resin into a mold, provided independently of each other, known is a screw preplasticating injection molding machine. In the front of the injecting plunger of the injecting unit, formed is a chamber, what is called a reservoir, for reserving a resin in an amount equal to the injection quantity of the resin per shot. To the chamber, the resin plasticated in the plasticating unit is supplied, and injected into the mold by moving the injecting plunger forward.
In such a conventional injection molding machine, the molding cycle comprises the processes of injection, dwelling, cooling, mold opening and closing, which are repeated in that order, as understood in the molding cycle of a conventional injection molding machine shown in FIG. 1. During the processes of injection, dwelling, and mold opening and closing, the plasticating unit is stopped. Simultaneously at the time when the cooling process is started, the screw of the plasticating unit is driven to plasticate the resin and feed the plasticated resin to the reservoir (measured).
Margin means a spare time till the opening and closing of the mold is started after the cooling, and is provided to stabilize the molding cycle. The period of the margin is changed depending on the injection amount of a resin per shot.
As described above, in the conventional injection molding, the plasticating process is carried out synchronously with the measuring process in the injecting unit and intermittently during a series of the processes constituting the molding cycle. As regards a resin such as LCP (liquid crystal polymer) which has low screw-engaging properties, the plastication does not start soon after the screw starts. Therefore, the plastication efficiency of the intermittent driving is very low. As a result, for the plastication, a long period of time is required. This brings about the prolongation of the molding cycle.
Accordingly, it is an object of the present invention to provide a thermoplastic resin injection molding machine of which the plastication efficiency of a resin can be enhanced, and the molding cycle can be shortened.
The above-described object can be achieved according to the present invention. According to a first aspect of the present invention, there is provided an injection molding machine which comprises a plasticating unit for plasticating a thermoplastic resin, an injecting unit connected to the plasticating unit through a connecting passage to inject the plasticated resin into a mold, and a buffering unit provided in the connecting passage to reserve the resin plasticated in the plasticating unit in an amount at least equal to the injection quantity of the resin per shot and feed the resin into the injecting unit.
According to a second aspect of the present invention, there is provided a thermoplastic resin injection molding machine which comprises a plasticating unit for plasticating a thermoplastic resin, and an injecting unit connected to the plasticating unit through a connecting passage to inject the plasticated resin into a mold, the plasticating unit comprising a cylinder, a screw rotatable and movable in the axial direction in the cylinder, means for rotation-driving the screw, a buffering chamber defined by the top portion of the screw and the cylinder to reserve the plasticated resin in an amount at least equal to the injection quantity of the resin per shot, and means of energizing the screw forward in the axial direction to feed the resin in the buffering chamber into the injecting unit.
The resin plasticated in the plasticating unit is fed to the injecting unit through the connecting passage. However, while the injection and the dwelling are carried out, the resin can not be fed. According to the first aspect of the present invention, the buffering unit is provided in the connecting passage connecting the plasticating and injecting units to each other. The plasticated resin is temporarily reserved in the buffering unit. Then, when the measuring is carried out, the reserved resin is fed into the injecting unit. As seen in the embodiment of the present invention of FIG. 1, the plasticating unit can be continuously driven independently of the movement of the respective parts of the molding cycle. Therefore, even for a resin having low engaging-properties such as LCP or the like, the plasticating efficiency can be enhanced. As a result, the molding cycle can be significantly reduced as compared with the conventional injection molding machine.
For the purpose of enhancing the production efficiency of injection molding, there are available two methods. In one method, the number of molding products per shot is increased, and in the other, the molding cycle is shortened. In the conventional molding, the shortening of the molding cycle has a limitation. Accordingly, it has been dominant that the number of molding products per shot is increased, that is, large-size molds are used for the enhancement of the production efficiency. However, this method has the problem that since the depreciation expense of the mold is high, the unit cost of the products becomes high especially in the production of a variety of products in small lots which has been increasingly employed in recent years.
On the other hand, in the case that an injection molding machine having a shortened molding cycle such as the injection molding machine of the present invention is employed, such a high production efficiency as comparable to that of a large-size mold can be secured for even a small-size mold. Accordingly, as compared with the conventional production system, significant cost-saving can be achieved with respect to mold depreciation expenses. Further, since the small-size mold of which the number of molding products per shot is small is employed, the required mold clamping force is decreased. As a result, advantageously, the mold clamping mechanism and the molding machine itself can be reduced in size, and moreover, the molding machine itself can be formed inexpensively.
In the thermoplastic resin injection molding machine according to the first aspect of the present invention, preferably, the buffering unit comprises a pot for reserving a plasticated resin, a plunger disposed in the pot so that it can be moved forward and backward in the pot, and means for energizing the plunger in the resin extrusion direction. The resin pressure in the pot can be controlled by use of the energizing force of the energizing means.
According to the first aspect of the present invention, the buffering unit is provided in the connecting passage. On the other hand, according to the second aspect of the present invention, the buffering unit is contained in the plasticating unit. That is, the buffering unit is contained in the unit for plasticating a thermoplastic resin. In particular, the injection molding machine comprises a plasticating unit for plasticating a thermoplastic resin, and an injecting unit connected to the plasticating unit through a connecting passage to inject the plasticated resin into a mold, the plasticating unit comprising a cylinder, a screw rotatable and movable in the axial direction in the cylinder, means for rotation-driving the screw, a buffering chamber defined by the top portion of the screw and the cylinder to reserve the plasticated resin in an amount at least equal to the injection quantity of the resin per shot, and means of energizing the screw forward in the axial direction to feed the resin in the buffering chamber into the injecting unit.
In this case, in addition to the advantages of the injection molding machine according to the first aspect of the present invention, the machine can be reduced in size, since the buffering unit can be integrated with the plasticating unit, and also, the deterioration of the resin is inhibited, since the resin earlier plasticated is fed earlier to the injecting unit, that is, in the order of plastication, without the plasticated resin being retained in the buffering chamber.
As the energizing means, a spring may be employed. Also, a fluid-pressure cylinder (pneumatic or hydraulic) may be used. Furthermore, an electric actuator is available.
When the spring is employed, the buffering unit can be simplified, and moreover, the reserving and feeding operation of the buffering unit can be automatically carried out without additional control being necessitated.
When the fluid-pressure cylinder is used, the resin extruding force of the plunger or screw can be desirably controlled. In addition, in the measuring process, the measuring period can be shortened by operating the fluid-pressure cylinder rapidly to feed the resin into the injecting unit at a high speed.
When the electric actuator is used, the resin extruding force can be desirably controlled, and moreover, the movement variable of the plunger or screw can be controlled at a high precision. As the electric actuator, a linear motor, a combination of a motor and a ball screw mechanism, or the like may be employed.
When the fluid-pressure cylinder is used as the energizing means, preferably, a constant fluid pressure from a fluid-pressure source is transmitted to the fluid-pressure cylinder. Thereby, the extruding force for the resin extruded from the buffering unit can be made constant without additional adjustment being necessitated. That is, the resin pressure in the reservoir can be made constant. As a result, dispersions in qualities of molding products can be inhibited.
Preferably, the injection molding machine further comprises a pressure sensor for detecting a resin pressure in the pot, and resin-pressure controlling means for controlling the energizing means correspondingly to the value detected by the pressure sensor so that the resin pressure is kept substantially constant. In this case, the extruding pressure for the resin to be extruded from the buffering unit can be made constant by feedback-controlling the fluid-pressure cylinder or electric actuator correspondingly to the value detected by the pressure sensor.
Also preferably, the injection molding machine further comprises a position detecting sensor for detecting a displacement of the plunger, and plastication-controlling means for controlling the plasticating unit correspondingly to the displacement. In this case, the amount of the resin reserved in the pot is calculated based on the value detected by the position detecting sensor, and the plasticating unit can be controlled so that the amount of the resin is prevented from exceeding the upper limit of the amount of the resin capable of being reserved in the pot.
Preferably, the structure of the injection molding machine further comprising the pressure sensor and the resin-pressure controlling means is applied to the injection molding machine according to the second aspect of the present invention. Furthermore, the structure of the injection molding machine further comprising the position detecting sensor and the plastication-controlling means is applied to the injection molding machine according to the second aspect of the present invention. In these cases, the same operation and advantages as those of the above-described structures can be attained, respectively.
The plasticating unit can be driven by an arbitrary driving system. However, according to the present invention, it is preferable that the plasticating unit carries out the plastication of the resin continuously during all the period of the molding cycle. Thereby, even a resin having low engaging-properties such as LCP can be efficiently plasticated. In this case, the plasticating unit is continuously driven, while the injecting unit is done intermittently. Accordingly, a pulsation is generated between them. However, since the pulsation is absorbed by the buffering unit, injection can be carried out with high stability.